Methods and apparatus for optical networks

ABSTRACT

A chromatic dispersion compensation system for an optical transmission system incorporates circuitry which determines the length of an optical fiber extending between an output amplifier and an input amplifier. Based on fiber type, the total chromatic dispersion on the fiber is determined. Compensation can then be automatically implemented.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of the filing date of U.S.Provisional Application Ser. No. 60/798,526 filed May 8, 2006 andentitled “Methods and Apparatus for Optical Networks”. The '526application is incorporated herein by reference.

FIELD

This invention relates in general to the field of optical networks.Various aspects of the present invention find application in the fieldof optical networks including, by way of example, networks that make useof wavelength division multiplexing technologies.

BACKGROUND

Wavelength Division Multiplexing (WDM), including without limitationDense Wavelength Division Multiplexing (DWDM), are techniques thatenable a multitude of optical wavelengths of differing frequencies to betransported over a single optical fiber. A DWDM network, for example, isconstructed by interconnecting multiple DWDM network elements usingglass optical fiber.

One limitation of maximum transmission distance over an optical fiber ischromatic dispersion, or spreading of optical pulses as they travelalong the fiber. For single-mode optical fiber, chromatic dispersionoccurs because the index of the glass varies slightly depending on thewavelength of the light, and the light associated with opticaltransmitters have nonzero spectral width. In order to combat the effectsof chromatic dispersion, chromatic dispersion compensation devices andmethods have been developed.

In known DWDM systems, chromatic dispersion compensation is oftenperformed on each input fiber of each network element. Chromaticdispersion compensation can be carried out by using dispersioncompensation fibers (DCF) or optical tunable dispersion compensators(OTDC). There continues to be a need for more cost effective solutionsto the problem of chromatic dispersion in optical transmission systems.Preferably such compensation could be achieved by incorporatingstandardized compensation modules into optical network elementsirrespective of length of the associated optical fiber. Further, itwould be desirable if compensation processing could be carried out inreal-time without any interruption of the flow of traffic in therespective optical fiber.

SUMMARY OF THE INVENTION

In accordance with the invention, chromatic dispersion of an opticalfiber can be automatically compensated. Length establishing circuitrycan automatically establish a length parameter of a respective opticalfiber. Determination circuitry, responsive to the established lengthparameter can automatically determine chromatic dispersion of the fiber.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram of two fiber links and associated Network Elementsaccording to one exemplary embodiment of the present invention;

FIG. 2 is a diagram of a non-measurement mode of operation according toone exemplary embodiment of the present invention;

FIG. 3 is a diagram of a measurement mode of operation according to oneexemplary embodiment of the present invention;

FIG. 4 is the flow diagram illustrating determining the total chromaticdispersion on a fiber between two Network Elements according to oneexemplary embodiment of the present invention;

FIG. 5 is a block diagram of the “CD Measurement Logic” blocks accordingto one embodiment of the present invention;

FIG. 6 is a diagram of a state machine within the “CD Measurement Logic”blocks according to one embodiment of the present invention;

FIG. 7 is a diagram from a Telcordia document GR-253-CORE illustratingexemplary transport overhead assignment for an OC-3 signal carrying anSTS-3c Synchronous Payload Envelope (SPE);

FIG. 8 is a diagram as in FIG. 7, illustrating use of two normallyunused bytes in the overhead to carry proprietary information accordingto one embodiment of the present invention;

FIG. 9 is a timing diagram depicting the insertion of loop back codewords according to one exemplary embodiment of the present invention;and

FIG. 10 is a timing diagram depicting the measurement delays accordingto one exemplary embodiment of the present invention.

DETAILED DESCRIPTION

While embodiments of this invention can take many different forms,specific embodiments thereof are shown in the drawings and will bedescribed herein in detail with the understanding that the presentdisclosure is to be considered as an exemplification of the principlesof the invention, as well as the best mode of practicing same, and isnot intended to limit the invention to the specific embodimentillustrated.

Methods and apparatus which embody the present invention provideimproved techniques for chromatic dispersion identification and/orcompensation. Since the amount of chromatic dispersion associated withany given fiber depends upon the length of fiber and the type of fiber,an appropriate amount of compensation can be applied to the fiber oncethe length and type of fiber is determined.

In accordance with a disclosed embodiment of the invention, a value thateither directly or indirectly corresponds to the length of the fiber atissue can be used to determine the amount of chromatic dispersion of thefiber and/or the appropriate amount of dispersion compensation for thefiber. Preferably, this value is derived directly from a measured lengthof the fiber, and is used in conjunction with information related to thetype of fiber at issue to make the chromatic dispersion determination.

Methods and apparatus which embody the present invention canautomatically derive one or more values that correspond to fiber lengthin real-time during normal operation. The amount of chromatic dispersionof an optical fiber and/or the appropriate amount of dispersioncompensation for that fiber can then be efficiently and effectivelydetermined concurrently with normal traffic transmission.

In one aspect of the invention, fiber length measurements can be madewithout interrupting the flow of traffic over the fiber being measured.Moreover, fiber measurements can be initiated immediately uponinstallation of DWDM network transmission elements, or any timethereafter. Preferably, measurement modules will be incorporated intoDWDM network elements.

FIG. 1 is a block diagram illustrating an exemplary embodiment 10 of thepresent invention. Two optical fibers 12, 14 are coupled by associatedinterface circuitry 12 a, b, 14 a, b between two connected NetworkElements 1, 2. Each Network Element, such as Network Element 1 includesboth an input and output amplifier 16 a, b, two optical supervisorychannel (OSC) filters 16 c, d, an OSC optical receiver 16 e, an OSCoptical transmitter 16 f, a dispersion compensation element 16 g, CDmeasurement logic 16 h, input and output DWDM filtering units (notshown), an OSC communication processor 16 i and a system processor 16 jboth of which are of a type that would be known to those of skill in theart. Network Element 2 includes corresponding elements 18 a-18 j aswould be understood by those of skill in the art.

In one embodiment of the present invention, the CD Measurement logic 16h, 18 h of FIG. 1 performs a Chromatic Dispersion measurement. Themeasurement may be initiated at Network Element 1 or at Network Element2.

The measurement can be initiated at Network Element 1 by activating a“Start Measurement” signal 20, from processor 16 j within NetworkElement 1. Once the measurement that is initiated at Network Element 1is complete, the measurement results are available to processor 16 j viaa signal 22 labeled “Measurement Results” within Network Element 1. In asimilar manner, the Chromatic Dispersion measurement may be initiated atNetwork Element 2 via a corresponding signal 20-1.

The OSC filter, for example 16 d, 18 d at the output of each outputamplifier 16 b, 18 b is used to combine the transport informationcarrying DWDM signal with an optical supervisory signal such as 24,24-1. An OSC filter, such as 16 c, 18 c at the input of each inputamplifier 16 a, 18 a is used to separate the optical supervisory channelfrom the transport carrying DWDM signal.

The optical supervisory signal is normally used to carry network levelcontrol information from one network element to another. In anon-measurement mode of operation, the CD measurement logic 16 h, 18 hcouples the OSC Transmit signal input to the Processed OSC Transmitsignal output 24, 24-1 (without modification), and couples the ProcessedOSC Receive signal input signal 26, 26-1 to the OSC Receive signaloutput (without modification).

A non-measurement mode of operation is illustrated in FIG. 2. In FIG. 2,the OSC signal path from Network Element 1 to Network Element 2 isillustrated via a dotted line indicated by 201. In FIG. 2, the OSCsignal path from Network Element 2 to Network Element 1 is illustratedvia a dotted line indicated by 202.

FIG. 3 illustrates the measurement operation for the case where ameasurement is initiated at Network Element 1. For this case, the OSCTransmit signal in Network Element 1 is modified such that a “far-endloop back” code word is inserted into the OSC Transmit signal viacircuitry or logic 303, and the resulting signal (Processed OSC Transmitsignal), signal 24, is forwarded to Network Element 2 via path 301. Whenthe “far-end loop back” code is received by circuitry 305 at NetworkElement 2, Network Element 2 loops back the Processed OSC Receive signalto the Processed OSC Transmit signal, as shown in FIG. 3 via signal 24-1and path 302.

In addition to performing the loop back operation, Network Element 2also inserts a “loop back performed” indicator into the Processed OSCTransmit signal via circuitry 305. After Network Element 1 determinesthat the loop back has been performed, it performs a measurement of thetime it takes for a signal to propagate from Network Element 1 toNetwork Element 2 and back to Network Element 1 (i.e., the round trippropagation delay).

Once the time measurement is complete, the total Chromatic Dispersion ofthe fibers between Network Element 1 and Network Element 2 can bedetermined using the processing illustrated in flow diagram 100 of FIG.4.

Two tables are incorporated in the flow diagram 100 of FIG. 4. One tablestores the effective group index of refraction of various fiber types at1510 nm (the wavelength of the OSC Channel). The other table stores thechromatic dispersion constants and dispersion slopes of various fibertypes at 1545 nm (assuming C-Band operation).

In one embodiment of the present invention, the “CD Measurement Logic”blocks 16 h, 18 h of FIG. 1 implement the processing of the flow chart100 of FIG. 4. In another embodiment of the present invention, only theround trip delay measurement is performed by the “CD Measurement Logic”blocks shown in FIG. 1, with the remaining steps being performed by aprocessor external to the “CD Measurement Logic” blocks. For example,processors 16 j, 18 j could carry out the remaining steps and determinethe compensation required (which can be provided by elements 16 g, 18 g)based on a determination of chromatic dispersion. Other processingvariations come within the spirit and scope of the invention. Thelocation of such processing is not a limitation of the invention.

The round trip chromatic dispersion determination, as described herein,assumes that the two fibers between a given set of Network Elements,such as Network Element 1, 2 are substantially the same length. It alsoassumes that the type of fiber between a given set of Network Elementsis known. These assumptions are consistent with the vast majority ofapplications.

In accordance with a preferred example embodiment of the presentinvention, the determination of chromatic dispersion represented by theflow chart of FIG. 4, can be used, in turn, to appropriately compensatefor the chromatic dispersion. By way of example and illustration, suchcompensation may be realized through selective tuning of an OTDC,elements 16 g, 18 g. Such tuning would be based upon the determinedamount of chromatic dispersion. Control can be implemented using thecircuitry which makes the chromatic dispersion determination (forexample processor 16 j or 18 j without limitation).

With respect to processing 100 of FIG. 4, in Step 102 the round trippropagation delay between two Network Elements is measured. In Step 104the one way propagation delay between the Network Elements iscalculated. In Step 106 the effective group index of refraction n isretrieved from Table 1. In Step 108 the distance d is calculated betweenthe Network Elements. In Step 110 a chromatic dispersion constant, basedon fiber type, is obtained from Table 2. Finally, in Step 112 the totalamount of chromatic dispersion on the fiber between the two NetworkElements is calculated. Compensation can then be automatically carriedout.

Those of skill will understand that a variety of measurement processescan be used to determine the length of a respective optical fiber. Allsuch variations come within the spirit and scope of the presentinvention. For example, transit time in one direction along a fiber canbe determined. Chromatic dispersion for that segment of fiber can beautomatically determined as disclosed above. The fiber can then beautomatically compensated.

Alternately, measurement indicia can be transmitted bi-directionally onthe same fiber. A length determination of that singular fiber can thenbe determined in accordance with the above. Chromatic dispersion can beestablished for that fiber based on the roundtrip measurement of thesingular fiber. That fiber can then be automatically compensated.

FIG. 5 illustrates one possible implementation of the CD MeasurementLogic 16 h, 18 h of FIGS. 1-3. This implementation assumes the use of aSONET based OSC channel, although other formats come within the spiritand scope of the invention. Those of skill in the art will recognize thepurpose and function of various SONET-type signals in FIG. 5. Suchsignals except to the extent noted subsequently need not be discussedfurther.

The implementation illustrated in FIG. 5 includes the transmit CDmeasurement circuitry 303, the receive CD measurement circuitry 304 ofthe measurement initiator (Network Element 1 in FIG. 3), and the loopback circuitry 305 of the non-initiator (Network Element 2 in FIG. 3).Therefore, if the modular circuitry shown in FIG. 5 is incorporated inthe CD Measurement Logic 16 h, 18 h of both Network Element 1 and 2,then either Network Element 1 or Network Element 2 can initiate ameasurement and carry out any needed compensation.

FIG. 5 also illustrates exemplary measurement circuitry 308. Circuitryis present within the CD measurement logic 308 to handle the case whereboth Network Elements 1 and 2 initiate measurements simultaneously.

Relative to FIG. 5, transmit circuitry 303 includes Clock and DataRecovery element 3-10, SONET Framer 3-12, Frame Timer 3-14, TX timerdecoder 3-16, Selector 1 element 3-18, SONET Scrambler and Serializer3-20 and Selector 2 element 3-22. The output signal from Selector 2,element 3-22 is the processed OSC transmit signal (24, 24-1) which iscoupled from measurement circuitry 16 h, 18 h to a respective opticaltransmitter 16 f, 18 f.

The receive circuitry 304 includes Clock and Data Recovery element 4-10,SONET Framer 4-12, RX Timer Decoder 4-16, Selector 4 element 4-18, andSONET Scrambler and Serializer 4-20.

Measurement circuitry 308 includes a Roundtrip Delay Timer 8-10, aHoldoff Timer 8-12 and control circuitry implemented as a finite statemachine 8-14. It will be understood that the state machine 8-14 could beimplemented with a programmable processor, 8-14 a in combination withexecutable control software 8-14 b. Other implementations come withinthe spirit and scope of the present invention. State machine 8-14 couldalso be implemented using interconnected hardwired circuit components.

Assuming that Network Element 1 initiates the measurement (as depictedin FIG. 3), Clock and Data Recovery (CDR) element 3-10 is used to firstrecover clock and data from the composite OSC Transmit Signal enteringthe CD measurement logic 16 h from processor 16 i, 18 i. The recoveredclock and data are then forwarded to the SONET Framer Descrambler andDe-Serializer element 3-12. The Framing bytes within the OSC SONETsignal are first located. Once these bytes are located, the signal isdescrambled and converted to a parallel “word” oriented signal.

The SONET Framer element 3-12 then generates a synchronization signal(TSYNC) which causes the TX SONET Frame Timer 3-14 to synchronize withthe incoming SONET data stream. The SONET Frame Timer 3-14 is used toidentify when certain word periods are appearing on the SONET datastream (via the TX Timer Decoder 3-16).

The SONET Frame Timer 3-14 is used to clock the Hold Off Timer 8-12within the Measurement Circuitry 308. The Hold Off Timer 8-12 is clockedonce at the end of every SONET frame period (i.e., every 125 microseconds).

Selector 1 element 3-18 in FIG. 5 is used to insert the normal SONETtransport overhead, the normal SONET data (Tdata), a selected loop backcode word, or the Measurement Marker (MM) into the Processed OSCTransmit signal output data stream. The SONET Scrambler and Serializer3-20 scrambles the data according to the SONET scrambling algorithm andthen serializes the parallel words received signals from the Selector 1element 3-18.

Selector 2 element 3-22 is used to either insert the OSC Transmit signaldata into the Processed OSC Transmit signal output stream, or to loopback the Processed OSC Receive signal into the Processed OSC Transmitsignal output stream. When the respective module 16 h, 18 h is acting asthe measurement initiator, the OSC Transmit signal data is inserted intothe Processed OSC Transmit signal output stream, signal 24-1. When theother module 18 h or 16 h is acting as the measurement non-initiator,the Processed OSC Receive signal is looped back into the Processed OSCTransmit signal output stream as illustrated in FIG. 3 by circuitry 305.

At the start of an initiated measurement, the “Send Loop Back Code Word”signal 8-20 is activated and sent from the CD Measurement state machine8-14 to the TX Timer Decoder circuitry 3-16. The activation of thissignal causes the CD Measurement Logic 16 h, 18 h to insert the loopback code word into the Processed OSC Transmit Signal output stream atthe appropriate time. This code word gets transmitted once per SONETframe period until the measurement is complete.

Once the loop back code word leaves Network Element 1, it propagatesalong the fiber, such as fiber 12, to Network Element 2. When NetworkElement 2 receives the loop back code word, it loops back its ProcessedOSC Receive signal 26-1 to its Processed OSC Transmit signal 24-1 viaits Selector 2 element 3-22. It then inserts the “Loop Back Performed”code word into the OSC data stream heading back towards Network Element1. In one embodiment of the present invention, the “Loop Back Performed”code word is placed into the same word timeslot as the loop back codeword. The “Loop Back Performed” code word has a different value thanthat of the loop back code word.

Once inserted into the Processed OSC Transmit signal at Network Element2, the “Loop Back Performed” code word propagates along the fiber, suchas fiber 14, to Network Element 1. The process of Network Element 2looping back the SONET signal from Network Element 1 back to NetworkElement 1 will in nearly all cases cause the SONET Framer 4-12 withinthe Receive Circuitry 304 of Network Element 1 to go out of frame.Therefore, in order to prevent false measurement readings, a “hold off”period is entered once Network Element 1 starts sending the loop backcode word. This time period is greater than twice the maximum round tripfiber delay period plus the time for a SONET framer to go out and backinto frame. Network Element 1 uses the Hold Off Timer 8-12 withinMeasurement Circuitry 308 in order to perform this function.

As previously mentioned, the Hold Off timer 8-12 is clocked once every125 microseconds. For round trip times on the order of 2 ms(approximately 400 km), plus allowing 1 ms for the framing processes,the Hold Off timer preferably includes a 5 bit counter (2⁵×125 us=4 ms).Once the Hold Off Timer 8-12 reaches its terminal count, a check is madeto determine if the SONET Framer 4-12 within the Receive Circuitry 304of Network Element 1 is in-frame. If it is in frame, then the CDMeasurement Logic 8-14 within Network Element 1 waits for the next loopback code word time slot to occur within the Processed OSC Receivesignal data stream. Once this occurs, if the “Loop Back Performed”indicator is not within the loop back code word time slot, themeasurement is aborted, and an error flag is set within Network Element1.

If the “Loop Back Performed” indicator is located within the loop backcode word time slot, then Network Element 1 inserts the MeasurementMarker word into its Processed OSC Transmit signal at the appropriatetime, and starts the Round Trip Delay Timer 8-10 (via the signal ResetRTDT) within its Measurement Circuitry 308.

Once Network Element 1 sends the Measurement Marker, it waits for thereturn of the Measurement Marker on its Processed OSC Receive signal.Once it receives the looped back Measurement Marker, its Round TripDelay Timer 8-10 is stopped. The content of the Round Trip Delay Timer8-10 now represents the amount of time it takes a signal to propagatefrom Network Element 1 to Network Element 2 and back to Network Element1.

Once the measurement is complete, Network Element 1 stops sending theloop back code word. This results in Network Element 2 being taken outof its loop back mode. Subsequently, in accordance with the processing,steps 104-112 of FIG. 4, the degree of chromatic dispersion can bedetermined.

Exemplary operation of the CD Measurement Logic State Machine 8-14 isillustrated in FIG. 6. Circuitry 8-14 transitions from initial state Ato state B in response to the start measurement signal 20. Circuitry8-14 transitions from state B to state C at a time to insert the loopback code. (When the loop back code word is inserted, the Hold Off timeris reset.) It transitions from state C to state D when the Hold Offtimer 8-12 has timed out. It transitions from state D to state E inresponse to receiving a loop back code word time indicator and anindication that the loop back performed code word has been received.Alternately, it transitions from state D to state J where the loop backcode word time indicator has been received and where the loop backperformed code word has not been received.

State machine 8-14 transitions from state E to state F when it is timeto insert the measurement marker (MM). (At this time, the Round TripTimer 8-10 is started via the signal Reset RTDT.) It transitions fromstate E to state J when the far end is no longer in a loop back mode. Ittransitions from state F to state G in response to a measurement markerbeing received on the RX line. It transitions from state F to state J inresponse to receiving an indication that the far end is no longer in aloop back mode, or, that the Roundtrip Delay Timer 8-10 terminal counthas been reached. It transitions from state G to state A once themeasurement has been completed.

It transitions from state B to state I in response to having received aloop back code.

It transitions from state A to state H in response to having received aloop back code. (This is the normal path of the non-initiator networkelement.)

It transitions from state H to state I in response to having received astart measurement signal 20.

In summary, the initiator network element would normally follow the pathfrom A to G to A, and the non-initiator would normally follow the pathfrom A to H to A. If errors occur during the measurement process eitherstate I or state J are entered, and the “measurement failed” flag isset.

Telcordia Technologies develops and markets communications networksoftware. FIG. 7 is a diagram from publicly available Telcordia documentGR-253-CORE illustrating exemplary transport overhead assignment for anOC-3 signal carrying an STS-3c Synchronous Payload Envelope (SPE). Sucha signal can be used to carry an OSC signal in accordance with thepresent invention.

FIG. 8 is the same diagram as FIG. 7, except that two unused overheadbytes are used to carry proprietary information bytes according to oneembodiment of the present invention. In FIG. 8, the loop back code word(CW) and the Measurement Marker word (MM) are 8-bit words and they areinserted into the two unused overhead bytes after the B1 byte. Otheroverhead locations or code assignment come within the spirit and scopeof the invention.

FIG. 9 illustrates a timing diagram depicting the insertion of loop backcode words, “Loop Back Performed” words and Measurement Marker wordsaccording to one embodiment of the present invention. In this particularembodiment, the loop back code word is 8-bits in length and has thevalue 55 hexadecimal. The Measurement Marker word is 8-bits in lengthand has the value 99 hexadecimal. The “Loop Back Performed” word is8-bits in length and has the value AA hexadecimal.

FIG. 10 illustrates a timing diagram depicting the measurement delaysaccording to one embodiment of the present invention. In FIG. 10, theactual time measured by the round trip delay timer is equal to thepropagation delay associated with the fibers in each direction plusinternal logic delays. Since the internal logic delays are known, theround trip delay (t_(RTD)) can be calculated from the measured delay(t_(MD)) by subtracting the internal delays from the measured delay.

Embodiments of the present invention to the extent that may beimplemented in software may include an article of manufacture on amachine accessible or machine readable medium having instructions. Theinstructions on the machine accessible or machine readable medium may beused to program a computer system or other electronic device. Themachine-readable medium may include, but is not limited to, floppydiskettes, optical disks, CD-ROMs, and magneto-optical disks or othertype of media/machine-readable medium suitable for storing ortransmitting electronic instructions.

The techniques described herein are not limited to any particularsoftware configuration. They may find applicability in any computing orprocessing environment. The terms “machine accessible medium” or“machine readable medium” used herein shall include any medium that iscapable of storing, encoding, or transmitting a sequence of instructionsfor execution by the machine and that cause the machine to perform anyone of the methods described herein. Furthermore, it is common in theart to speak of software, in one form or another (e.g., program,procedure, process, application, module, unit, logic, and so on) astaking an action or causing a result. Such expressions are merely ashorthand way of stating that the execution of the software by aprocessing system causes the processor to perform an action to produce aresult.

In the foregoing description, the invention and various aspects thereofare described with reference to example embodiments. The specificationand drawings are accordingly to be regarded in an illustrative, ratherthan restrictive, sense. It will be evident to those skilled in the artthat various modifications and changes may be made to the exampleembodiments disclosed herein without departing from the broader spiritand scope of the present invention.

1. A control system comprising: an optical fiber; an optical filter onopposing ends of the optical fiber, said optical filter having a firstport that couples with the optical fiber, a second port that couples anoptical supervisory channel with the optical fiber and a third port thatcouples a transport channel with the optical fiber and where the opticalfilter separates the optical supervisory channel from the transportchannel; at least one measurement module coupled to the optical fiberthrough the optical supervisory channel, the module includes circuitryto determine a transit time between first and second ends of a trafficcarrying optical fiber when in a first mode, the module also includescircuitry to loop back a received optical signal when in a second modeand, where the module includes one of a hardware timer or a softwaretimer to accumulate a representation of the transit time; and circuitryresponsive to the determined transit time, to determine a degree ofchromatic dispersion.
 2. A system as in claim 1 which includes amulti-state control element.
 3. A system as in claim 1 which includesadditional circuitry, responsive to the representation to establish adegree of chromatic dispersion.
 4. A system as in claim 3 which includesa dispersion compensation element coupled to the additional circuitry.5. An apparatus for compensating chromatic dispersion of an opticalfiber where the apparatus comprises: an optical supervisory channel; atransport channel: an optical filter on opposing ends of an opticalfiber, said optical filter having a first port that couples with theoptical fiber, a second port that couples the optical supervisorychannel with the optical fiber and a third port that couples thetransport channel with the optical fiber and where the optical filterseparates the optical supervisory channel from the transport channel;first and second spaced apart network elements, the elements are eachcoupled to a traffic carrying optical fiber through the opticalsupervisory channel; first and second substantially identicalmeasurement modules with the first module coupled to the first elementand the second module coupled to the second element, at least one of themodules includes one of a hardware timer or a software timer toaccumulate a representation of a round trip transit time of aMeasurement Marker word between the elements, the modules each includecontrol circuitry that receives in input signal to initiate a round tripmeasurement transmission between the elements while traffic is beingcarried between elements; and additional circuitry, responsive tocontents of the timer, to determine chromatic dispersion of the fiberbetween the elements.
 6. An apparatus as in claim 5 wherein the controlcircuitry also initiates operation of the timer, and responsive to areceived measurement transmission, a receiving module enters a loop backperformed indicator thereinto and returns the transmission to atransmitting module which in response thereto terminates operation oftimer; and at least one device that compensates chromatic dispersion,coupled to one of the elements.
 7. An apparatus as in claim 6 where eachmodule includes circuitry to enter a loop back indicium into themeasurement transmission to the other module and where the other moduleresponds thereto and enters the loop back performed indicator into thereceived transmission.
 8. An apparatus as in claim 7 where the chromaticdispersion determining circuitry includes a plurality of pre-storedvalues of effective group index of refraction.
 9. An apparatus as inclaim 7 where the chromatic dispersion determining circuitry includes atleast one pre-stored chromatic dispersion constant.
 10. An apparatus asin claim 7 where in the absence of the loop back indicium, the othermodule does not return the transmission to the transmitting module.